NEU Quiz Somatosensation Flashcards
Four Major Somatosensory
Modalities
- Discriminative touch (mechanosensation)
- Proprioception (awareness of own body position)
- Temperature (noxious(damage/painful) and non-noxious)
- Nociception (pain)
Mediated by distinct classes of responders and a lot of time located in specific sense organs and the purpose is transforming one type of stimuli (non painful touch or painful temp/ touch into electrical signals)
What are the types of primary afferents that send somatosensory information? Which information do each of them send?
Primary Afferents (peripheral axons that act as dendrites)
Primary afferents are sensory neurons (axons or nerve fibers) in the peripheral nervous system that transduce information about mechanical, thermal, and chemical states of the body and transmit it to sites in the central nervous system.
Myelinated:
Aα - proprioception (primary and very fast)
Aβ - mechanoreception (less quickly, non painful touch)
Aδ delta – pain and temperature (thinly myelinated sending pain and temp)
Unmyelinated:
C (fibers/axon) – pain, temperature, itch
Where are the cell bodies of somatosensory cells located?
Dorsal Root Ganglion (DRG)
Pseudounipolar cells - cell body - peripheral axon which is primary afferent - central axon/process (cell body to central nervous system) - periphery (receives info from periphery and acts as dendrite) - travels in one direct
Neuron cell body is located in the DRG adjacent to the spinal cord → this is technically in periphery
The axon of the DRG neuron has two branches, one enters from the periphery and one heads to the CNS
How do we sense stimuli in our
periphery?
Receptors of the Somatosensory System
Mechanoreceptors - non-noxious touch
Detect mechanical stimulation of skin or
other parts of body
– Change mechanical stimulation to
electrical signal
Two types:
– Sensory receptor cell
mechanoreceptor (like hair cell) –> special cells that respond to outside stimuli and send NT
– Sensory neuron mechanoreceptor
(specialized end of neuron) –> actual primary afferent
* Stretching of receptor cells opens
Na+ channels
Can “adapt” to stimulation:
– Rapid adaptation (signal start at end of stimuli)
– Slow adaptation (signal stays for entire length)
Types of Mechanoreceptors
Glabrous, superficial
layers (epidermis and
directly below epidermis) –> closer to skin
– Meissner’s corpuscles
– Merkel cells
Glabrous, deep layers
(dermis)
– Pacinian corpuscle
Merkel Cell
Deep in epidermis (1st layer)
– Fingertips
Features:
– Spatial resolution - small receptive fields
– Sensitive to indentation of skin
– Depth of indentation
* Edges
* Points
* Corners
* Curves
Signaling:
– Synaptic vesicles release NT - seretonin
– Cell innervated by primary afferent
neuron
– SLOWLY adapting - active for entire
Meissner and Pacinian Corpuscles (structure of end of afferent)
Meissner – Just beneath epidermis (1st layer close to surface)
Structure:
* Spiral nerve ending - specialized ends of primary afferent AND Buried in group of stacked lamellar cells
Features:
* Lower spatial resolution than Merkel cells, higher spatial resolution than Pacinian
* Rapidly adapting (good for timing/temporal
resolution) - active start and end of stimulis
* Respond to low frequency vibration (stroking suffeling)
Pacinian - Deep in dermis (base layer)
Structure:
* Nerve ending - specialized end of primary
– Buried in group of concentric lamellar cells
Features:
* Poor spatial resolution - deep
* Rapidly adapting (good for timing/temporal
resolution)
* Respond to high frequency vibration (earth quakes)
Where are the cell bodies of somatosensory cells located?
dorsal root ganglion
Receptive Fields
small receptive fields are stimulated by high spatial frequencies, fine detail; large receptive fields are stimulated by low spatial frequencies, coarse detail.
Which type of somatosensory afferents
transmit information from touch
mechanoreceptors to the central nervous
system?
a. A-alpha
b. Aβ
c. Aδ
d. C
b. Aβ
Merkel cell afferent fibers convey information
about which variable(s)?
a. Motion detection
b. Temperature
c. Proprioception
d. Shape and texture
e. Subtle vibrations
d. Shape and texture
Somatosensory neurons in the dorsal root
ganglion (DRG) transmit information
a. from the cell body to the spinal cord.
b. from the cell body to the skin.
c. from the skin to the spinal cord.
d. from the spinal cord to the skin.
e. from the spinal cord to the brain.
c. from the skin to the spinal cord.
Nociceptors
- Primary afferent preferentially sensitive to a
noxious stimulus (potienally or activly damaging stimuli) - Activated directly (temp + mechanical stimulation) OR indirectly (things relased by damaged tissue)
- Mechanical – high threshold, mostly Aδ (range of mechincal stimilu that are painful)
- Thermal - Aδ and C fibers (some respond to both temp and mechanical)
- Polymodal – C fibers (temp, mechanical chemical, inflammatory)
First pain is A-delta (immediate, sharp, fast)
Second pain is C fibers (throbbing, acing, slow)
Nociception - perifery spinal cord
Tissue injury damages cells, attracts
inflammatory cells (damaged associated molecular patterns that bind to C fibers which activate immune cells)
– Damaged/Inflammatory cells release
several substances:
* Histamine
* Arachadonic acid → prostaglandins
– Stimulated nociceptors release:
* Substance P
– Increases vasodilation,
inflammation, release of
histamine
– Excites interneurons(2*
afferents) in spinal cord
* Glutamate
– Excites interneurons(2*
afferents) in spinal cord
Pain Modulation
- Endogenous opioids: (brainstem neruons make)
– Enkephalin
– Endorphin
– Dynorphin - Opioid Receptors: (hypo, inhbit GPCR, k leave and blocks ca)
– 3 Types - Mu - Endorphin
- Delta - Enkephalin
- Kappa- Dynorphin
– Mechanism of action: - Bind opioid ligand
- Release inhibitory NT on nociceptor neuron terminals
– Involves Ca2+ and/or K+ channels
What is sensitization?
After acute injury or during chronic pain
Sensitization is a leftward shift in a stimulusresponse function: decrease threshold, increase in magnitude of response to a previously effective stimulus
Central Sensitization
- Neurophysiological changes that lead to
increased responsiveness to normal or subthreshold input - In dorsal horn these changes include
strengthening of synapses and disinhibition - Neuroinflammation in the dorsal horn promotes central sensitization
How are signals sent from the
periphery to the brain?
Somatosensory Pathways
Projections of Sensory Neurons
Neurons carry APs from skin to spinal cord
Spinal nerves:
* 8 Cervical
* 12 Thoracic
* 5 Lumbar
* 5 Sacral
* 1 Coccygeal
– “Mixed” nerves
* Contain both motor and sensory
neurons in periphery – contains
information sent from
mechanoreceptors and nociceptors
Somatosensory Organization – Types of
Neurons
First order(1*) – receive input and carry to spinal cord
– 2 Types of neurons:
* Dorsal root ganglion neurons –
everything but face
* Trigeminal sensory neurons of trigeminal
nerve -FACE
- Second order (2*-TRACTS) – carry input to
thalamus/medulla - Third order(3*) – Thalamus to cortex
Central Somatosensory Pathways:
The 3 neuron rule
For touch, temperature and pain:
It takes 3 neurons for a sensory signal to be relayed from the body to the
sensory cortex
PAIN/TEMP
1: Sensory Neuron
2: Spinal Neuron (crosses 2nd order) - DRG or Trig
3: Thalamic Neuron
PAIN/TEMP TOUCH
1: Sensory Neuron
2: Medulla Neuron (crosses) 2nd order- DRG or Trig
3: Thalamic Neuron
Sensation Pathway – Touch
Discrimination
Medial Leminiscal Pathway – BODY (DRG)
– Carries information about discriminative touch
– 1* afferents terminate in medulla (ipsilateral – same side):
– 2* afferents from dorsal column nuclei to dorsal thalamic
nuclei
* Decussate – cross over to contralateral (opposite) side
– 3* afferents from VPN to primary somatosensory cortex
parietal lobe
Trigeminal Sensory Pathway – FACE (Trig)
– Carries information about discriminative touch
– 1* afferents from trigeminal nerve terminate in ipsilateral
pons
– 2* afferents from trigeminal nucleus in pons to VPN
– 3* afferents from VPM to primary somatosensory cortex
Ascending Nociception Pathways
Anterolateral Pathway – 2 Parts:
Paleospinothalamic tract – SLOW
–To thalamus and parietal cortex
*From C fiber nociceptor axons
–To the reticular activating system
* Affects arousal, mood, attention
response
Neospinothalamic tract - FASTER
* To the thalamus and parietal cortex
* From Aδ fiber nociceptor axons
Both decussate (cross-over) to
contralateral side in the spinal cord!
Primary Somatosensory cortex
Located in parietal lobe
Somatotopic Organization
– Regions of body represented like a
map in cortex
Which primary afferents conduct the
sensation of second pain? What qualities are
typically associated with second pain?
a. Unmyelinated C; sharp and pricking
b. Myelinated A-delta; dull and aching
c. Unmyelinated C; dull and aching
d. Myelinated A-delta; sharp and pricking
e. Myelinated A-beta; sharp and pricking
c. Unmyelinated C; dull and aching
Which deficit would a lesion restricted to the right side of the spinal cord produce?
a. Pain and mechanosensory deficit on the left side of the body only
b. Pain and mechanosensory deficit on the right side of the body only
c. Pain deficit on the left side and mechanosensory deficit on the right side of the body
d. Pain deficit on the right side and mechanosensory deficit on the left side of the body
Pain deficit on the left side and mechanosensory deficit on the right side of the body
Which statement about primary somatosensory cortex
(S1) is true?
* a. It is organized based on the type of primary afferent carrying the information from the periphery to the second order neuron
* b. The amount of cortex dedicated to each body part is proportional to the surface area of the body part.
* c. It is organized by receptor type, with a Pacinian area, a Merkel disc area, a temperature area, and a pain area.
* d. The amount of cortex dedicated to each body part is proportional to the sensitivity of the body part.
d. The amount of cortex dedicated to each body part is proportional to the sensitivity of the body part.
Which primary afferents conduct the sensation of second pain? What qualities are typically associated with second pain?
a. Unmyelinated C; sharp and pricking
b. Myelinated A-delta; dull and aching
c. Myelinated A-delta; sharp and pricking
d. Unmyelinated C; dull and aching
e. Myelinated A-beta; sharp and pricking
d. Unmyelinated C; dull and aching
Which deficit would a lesion restricted to the right side of the spinal cord produce?
a. Pain and mechanosensory deficit on the left side of the body only
b. Pain and mechanosensory deficit on the right side of the body only
c. Pain deficit on the left side and mechanosensory deficit on the right side of the body
d. Pain deficit on the right side and mechanosensory deficit on the left side of the body
c. Pain deficit on the left side and mechanosensory deficit on the right side of the body
Discriminative touch is sent by _____ fibers. The second order neuron is located in the ________, where its axons then cross over.
A-beta
Medulla
Pain and temperature are sent by _______ and
__________. The second order neuron is located in the ______________, where its axons then cross over.
A-delta
C fibers
Spinal cord dorsal horn
Which statement about primary somatosensory cortex (S1)
is true?
* a. It is organized based on the type of primary afferent carrying the information from the periphery to the second order neuron
* b. The amount of cortex dedicated to each body part is proportional to the surface area of the body part.
* c. It is organized by receptor type, with a Pacinian area, a Merkel disc area, a temperature area, and a pain area.
* d. The amount of cortex dedicated to each body part is proportional to the sensitivity of the body part.
- d. The amount of cortex dedicated to each body part is proportional to the sensitivity of the body part.
